Surgical relief of left ventricular obstruction by left ventricular myotomy/myectomy or mitral valve replacement is a therapeutic option in patients with hypertrophic cardiomyopathy who are severely symptomatic and refractory to medical management. To determine the effects of surgical relief of left ventricular outflow obstruction in patients with hypertrophic cardiomyopathy, 20 patients were studied at rest and during atrial pacing before and after operation (septal myectomy in 14 and mitral valve replacement in 6). Coronary flow to the anterior left ventricle and septum, the site of maximum hypertrophy in these patients, was assessed by thermodilution. In all 20 patients there was successful relief of resting left ventricular outflow tract gradient from a preoperative gradient of 61+35 to 4+7 mmHg postoperatively. Surgical relief of left ventricular outflow tract obstruction significantly reduced left ventricular systolic pressure, coronary flow, and myocardial oxygen consumption at rest and during pacing, lowered left ventricular end-diastolic pressure following pacing, and improved anginal threshold and metabolic evidence of ischemia. These results demonstrate the importance of left ventricular outflow tract gradients in hypertrophic cardiomyopathy as well as the mechanism of improved effort tolerance after surgical relief of obstruction.